Ultrasound is currently used to generate images of the flesh and organs of the body. A standard imaging device includes a transmitter, composed of electronic circuitry and a piezoelectric transducer. The transmitter transmits acoustical waves into the body, and these waves travel through the flesh and organs, and cause echoes from every location where the density of the tissue changes, the tissue interfaces. Some of these echoes travel back towards the transducer and input to a receiver. The receiver includes the transducer previously mentioned (it is bi-directional), and electronic circuitry. The transducer, in the receiving mode, converts the echoes to electrical signals, and the receiving electronic circuitry amplifies these electrical signals.
The echoes from any one tissue interface have the shape of a mildly damped sine wave and thus, require several cycles to dissipate, or ring down. Due to the lengthy ring down period, a problem arises when two tissue interfaces are located close together. In this situation, the transmitted wave strikes the nearer tissue interface first and causes a protracted echo. Soon afterwards, the transmitted wave (which still has retained most of its energy) strikes the second interface and causes a second, protracted echo. The second echo travels back towards the first interface which is still ringing and producing an echo. Since the first interface is still producing an echo, the two echoes combine vectorily and make a waveform which is not characteristic of either tissue interface. The combined echoes cause a distorted image of the first and second tissue interfaces.
The prior art consists mainly of techniques to make the duration of the transmitted wave shorter since the tissue will begin to stop vibrating when the excitation wave ceases. One such technique is to electrically match the transducer.
Another technique is to utilize mechanical matching layers on the transducer to facilitate the flow of ultrasound through the transducer and to decrease reverberations.
Thirdly, the piezoelectric crystal or material can be produced so that the transducer has a high bandwidth.
However, the problem with these three techniques is that they do not decrease the ringing tendency of the tissue and so, are limited in their ability to shorten the duration of the echo. Also, these techniques do not reduce the ringing of the transducer enough.
Another prior art system, Kalman Filtering, involves: transmission of a standard, protracted ultrasonic wave, receipt of standard, overlapping or "convoluted" echoes, conversion of these convoluted echoes in the normal manner to electrical signals, and finally, use of a computer to process the electrical signals to "de-convolute" the electrical signals to resolve the closely-spaced tissue interfaces. Thus, Kalman filtering does not deviate from conventional process of transmission of waves and receipt of echoes; rather, it involves subsequent processing by a computer. The problem with Kalman Filter is that the computer is expensive and complicated, and the process has not yet proven effective in practice.